Flexible preparatory MR imaging

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-238772, filed Aug. 25, 1999, the entire contents of which are incorporated herein by reference.

This invention relates to a magnetic resonance imaging apparatus configured to use magnetic resonance (MR) to obtain a magnetic resonance image of a to-be-examined subject, and more particularly to a magnetic resonance imaging apparatus designed to perform a monitor scan for observing the flow of a contrast agent injected into a to-be-examiner subject, and perform an imaging scan for acquiring a photographic image when it is confirmed that the contrast agent has reached a diagnostic region in the subject.

In the field of magnetic resonance imaging apparatuses, several imaging methods suitable for particular diagnosis are proposed. Some MR contrast imaging methods use a function entitled “Visual Prep”. In MR contrast imaging methods using the Visual Prep function, a contrast agent is injected into a to-be-examined subject, and a monitor scan is executed. During the monitor scan, a plurality of monitor images are continuously generated and displayed on a monitor. The operator observes the flow of the contrast agent based on monitor images.

Upon confirming the arrival of the contrast agent at the desired diagnostic region, the operator inputs an instruction to execute an imaging scan. By the imaging scan, an MR contrast image to be used for diagnosis can be acquired at an appropriate point in time. A preparatory function that performs the monitor scan may be called a “Visual Prep” function.

The conventional technique using a Visual Prep function, however, is disadvantageous in that it cannot follow movement of the to-be-photographed subject (for example, movement of the human body), which occurs during the monitor scan. Specifically, in a conventional Visual Prep function, a mask image (a parent image) of the to-be-examined subject acquired at the initial stage, and a plurality of live images acquired at regular intervals are used. Each time a live image is acquired, the mask image and the live image are subjected to subtraction processing, and the resultant subtraction image is displayed on the monitor. Further, in a conventional Visual Prep function, the live image is updated at regular intervals, while the mask image is not updated but, instead, the initial mask image is used at all times. Therefore, when the to-be-examined subject moves, subtraction processing cannot be executed appropriately, resulting in an inappropriate monitor image.

Once the inappropriate monitor image is displayed, the operator cannot clearly observe the flow of contrast agent. This means that the operator may miss the desired timing for inputting an instruction to execute an imaging scan, and hence may fail to acquire a desired MR contrast image.

The present invention has been developed in light of the above, and aims to provide a magnetic resonance imaging apparatus which can more effectively display a monitor image showing the flow of contrast agent into a to-be-examined subject, thereby enabling the operator to determine appropriate timing for an imaging scan and hence more easily obtain a desired MR image.

According to the present invention there is provided an image display method employed in a magnetic resonance imaging apparatus that executes preparatory imaging, comprising the steps of: collecting a series of magnetic resonance images of a to-be-examined subject, which are to be used for preparatory imaging; determining at least one of the series of magnetic resonance images to be a parent image; creating a difference image between a latest one of the series of magnetic resonance images and the parent image; displaying the difference image on a monitor; updating the parent image in response to a first outside instruction by replacing the parent image with the latest image; displaying, on the monitor, the difference image based on the updated parent image; and collecting a photographic image in response to a second outside instruction.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrates presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a schematic block diagram showing a computer system and a display system, which are incorporated in a magnetic resonance imaging apparatus according to the present invention;

FIG. 2 is a view illustrating an example of a first screen for a Visual Prep function according to an embodiment of the present invention;

FIG. 3 is a view illustrating an example of a second screen for the visual prep function according to an embodiment of the present invention;

FIG. 4 is a flowchart useful in explaining the operation of an embodiment of the invention; and

FIG. 5 is a schematic block diagram illustrating the configuration of a magnetic resonance imaging apparatus.

An embodiment of the invention will be described with reference to the accompanying drawings.

The exemplary embodiment relates to a magnetic resonance imaging apparatus equipped with a so-called Visual Prep function. FIG. 5 is a schematic block diagram illustrating the configuration of the magnetic resonance imaging apparatus. As shown in the figure, a magnet assembly MA for receiving a to-be-examined subject P comprises superconducting or normal conducting static magnetic field coils 1 for generating a high-intensity static magnetic field, three gradient magnetic field coils 2 for generating gradient magnetic fields along the X-, Y- and Z-axes, a signal-sending/receiving coil 3 for excitation and signal collection, and a static magnetic field control system 4 for executing excitation control of the static magnetic field coil 1 and coolant supply control.

The X-, Y- and Z-axis gradient magnetic field coils 2 are controlled by X-, Y- and Z-axis gradient magnetic field power supplies 5 , 6 and 7 , respectively. The signal-sending/receiving coil 3 is driven by a transmitter 8 when sending a signal (excitation), and driven by a receiver 9 when receiving a signal (signal collection).

The magnetic resonance imaging apparatus comprises a sequencer 10 for causing the X-, Y- and Z-axis gradient magnetic field power supplies 5 , 6 and 7 , the transmitter 8 and the receiver 9 to execute sequential generation of gradient magnetic fields and sending/receiving signals, a computer system 11 for executing various processes relating to reconstruction of a magnetic resonance image, and a display unit 12 for displaying the magnetic resonance image created by the computer system 11 .

FIG. 1 is a schematic block diagram showing a computer system and a display system, which are incorporated in a magnetic resonance imaging apparatus according to the invention.

A computer system 11 shown in FIG. 5 has an image processing unit 30 and an image storing unit 40 . The image processing unit 30 includes an image reconstructing section 31 , a difference arithmetic operation section 32 and a display control interface 33 . The image storing unit 40 includes a reconstructed-image (live image) storing section 41 , a parent image (mask image) storing section 42 , a difference image (subtraction image) storing section 43 and a photographic image storing section 44 .

The image reconstructing section 31 reconstructs a magnetic resonance image of a to-be-examined subject on the basis of magnetic resonance image data concerning the subject output from the receiver 9 .

The difference arithmetic operation section 32 generates a difference image on the basis of a parent image stored in the parent image storing section 42 and a reconstructed image stored in the reconstructed-image storing section 41 , and stores it in the difference image storing section 43 .

The display unit 12 periodically reads reconstructed images stored in the reconstructed-image storing section 41 , or difference images stored in the difference image storing section 43 , thereby continuously displaying them on a monitor. Further, the display unit 12 displays a photographic image stored in the photographic image storing section 44 .

A rough description will now be given of the procedure of MR contrast imaging using the magnetic resonance imaging apparatus configured as above, and the Visual Prep function.

First, a contrast agent is injected into a to-be-examined subject by an operator, and a monitor scan is executed. On the basis of a monitor image obtained by the monitor scan, the operator observes the flow state of contrast agent. After confirming that the contrast agent has reached a to-be-photographed target, the operator inputs an instruction to execute an imaging scan, i.e. main scan for acquiring a photographic MR image used for diagnosis.

The Visual Prep function enables visual confirmation, based on the monitor scan, of the fact that contrast agent has reached a to-be-photographed target, and hence enables easy determination of timing for inputting an instruction to execute an imaging scan.

A monitor image displayed during the monitor scan is a normal reconstructed image or a difference image such as an absolute difference image or a raw-data difference image. The difference image is an image obtained by subjecting, to image difference processing (also called “subtraction processing”), a predetermined parent image (mask image) obtained earlier and a live image obtained at the present point in time. This image indicates only the contrast agent that is extracted as a result of removing a stationary subject by subtraction. As will be described later, the parent image can be updated by the operator at a voluntary point in time.

Gd-DPTA (gadolinium diethylenetriamine pentaacetic acid), for example, is known as a contrast agent to be injected into a subject. Contrast agents for MRI mainly serve (1) to impart contrast to tissue, (2) to make it easy to observe a tissue function, (3) to make it easy to observe a metabolic pathway, etc.

FIG. 2 is a view illustrating an example of a first screen for the Visual Prep function. The first screen is set on a display unit 12 when executing the Visual Prep function, and includes an image display area 21 and an operation button area 22 .

The image display area 21 displays a monitor image I 0 obtained via the computer system 11 . The monitor image I 0 is a normal reconstructed image or a difference image. Where a difference image is designated, the monitor image 10 is an absolute difference image or a raw-data difference image. The first screen can display only one image at a time, but the image is of a greater size than in a second screen described later.

The operation button area 22 is an area to be used by the operator to mainly input instructions to execute a scan, to change the displayed image, or to update the parent image, which is the feature of this embodiment. A plurality of buttons B 0 -B 7 are arranged in the operation button area 22 . These buttons can be clicked by the operator using, for example, a mouse (not shown), and include a parent image updating button B 0 , a normal reconstructed-image button B 1 , a difference image button B 2 , an absolute value image button B 3 , a raw-data difference image button B 4 , a monitor scan start button B 5 , an imaging scan start button B 6 and an imaging scan interrupt button B 7 .

When a screen switching button (not shown) has been operated at any particular point in time while the first screen is set on the display unit 12 , the screen is switched from the first screen to the second screen, which will now be described.

FIG. 3 shows an example of the second screen.

Like the first screen, the second screen is set on a display unit 12 when executing the visual prep function, and includes an image display area 21 and an operation button area 22 . The second screen differs from the first screen in that, in the former, a plurality of images can be displayed simultaneously in the image display area 21 .

The image display area 21 simultaneously displays a normal reconstructed image 11 , an absolute value difference image 12 , a raw-data difference image 13 and a positioning image 14 .

Since the second screen simultaneously displays a plurality of images, the operation button area 22 does not have any button for designating the image type of a to-be-displayed object. More specifically, the normal reconstructed-image button B 1 , the difference image button B 2 , the absolute value image button B 3 and the raw-data difference image button B 4 are not included. The button area only includes the parent image updating button B 0 , the monitoring scanning start button B 5 , the imaging scanning start button B 6 and the imaging scanning interrupt button B 7 .

When the aforementioned screen switching button has been operated at a voluntary point in time while the second screen is set on the display unit 12 , the screen is switched from the second screen to the first screen.

The operation of the embodiment constructed as above will be described.

FIG. 4 is a flowchart useful in explaining the operation of the embodiment.

When the operator has input an instruction to start MR contrast imaging using the Visual Prep function, first, initialization is executed at a step S 0 . During initialization, the first screen, for example, is set as a default, and an absolute value difference image is displayed as the monitor image I 0 on the first screen. If necessary, it is preferable that an image suitable for the monitor display is obtained in advance by executing contrast adjustment processing enlarging/reducing processing, etc.

Subsequently, the operator clicks the monitor scan start button B 5 on the first screen, thereby causing the MRI apparatus to start the monitor scan. A first image obtained upon the start of the monitor scan is stored as the parent image in the parent image storing section 42 (step S 1 ).

After that, a contrast agent is injected into a to-be-examined subject by an operator such as a doctor (step S 2 ).

Images (absolute value images) of the subject are created at regular intervals, and each of these images and the previously stored parent image are subjected to image difference processing, thereby obtaining and displaying a difference image on the display (step S 3 ). The displayed difference image only indicates the injected contrast agent since an image corresponding to the body of the subject is removed by the image difference processing. In the image difference processing, predetermined weighted processing may be executed on a to-be-processed image so as to obtain an appropriate image.

At the next step S 4 , it is determined whether or not a button relating to switching of display for different types of images has been clicked (pushed). If it is detected that the button B 1 has been clicked, the present screen for absolute value difference images is switched to a screen for normal reconstructed images. Further, if the button B 4 is clicked with the button B 2 already clicked, the present screen for absolute value difference images is switched to screen for raw-data difference images (step S 5 ).

This type of switching of screens can be repeated at any desired time point during the monitoring display. The processing at the step S 4 is skipped while the second screen is set.

The absolute value difference image is effective for the movement of a to-be-photographed subject, i.e. movement of the body of a to-be-examined subject. On the other hand, in the case of the raw-data difference (complex difference) image, a signal indicating the inflow of blood is positive and hence stable display is realized.

During the display of a monitor image, it is possible that the parent image used in present difference processing will become inappropriate because of movement of the body of the subject, breathing or inflow effect of blood. In such a case, the operator clicks the parent image updating button B 0 to thereby update the parent image.

It is determined at a step S 6 whether or not the operator has input an instruction to update the parent image. If the operator clocks the parent image updating button B 0 at a voluntary time point during the monitor image display, the stored parent image is replaced with an image presently obtained, thereby updating the parent image (step S 7 ). After that, a difference image based on the updated parent image is displayed. In a manner similar to the switching for different types of images, the parent image can be updated anytime.

Upon confirming from the monitor image that the contrast agent has reached a to-be-photographed target, the operator clicks the imaging scan start button B 6 . As a result, imaging scan is executed in place of the monitor scan, thereby obtaining a desired MR contrast image (step S 9 ). If the imaging scan interrupt button B 7 is clicked, the imaging scan is promptly interrupted, although the flowchart of FIG. 4 does not show it.

As described above, the exemplary embodiment realizes more effective display of a monitor image relating to the Visual Prep function. In other words, the parent image of a difference image can be updated at a voluntarily chosen time point, and further the type of a to-be-displayed image can be changed in accordance with, for example, a to-be-monitored portion of a subject body. On the basis of such a monitor image, the operator can clearly see the flow of contrast agent. Accordingly, the operator is less likely to fail to determine the right moment for instructing the apparatus to start a series of imaging operations, i.e. the time point of instructing the apparatus to execute photography. As a result, a desired MR contrast image can be more easily and reliably obtained. This means that the possibility of failure of MR contrast photography is low, and hence strain on the body of a to-be-examined subject can be reduced.

As described above, the present invention can provide a magnetic resonance imaging apparatus capable of effectively displaying a monitor image that indicates the flow of a contrast agent in a to-be-examined subject, thereby enabling the operator to more easily choose the timing for photography.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.